PALAEONTOLOGY OF NEW-YORK. 
3® 
The interior of the ventral valve preserves very strong dental lamella, 
and a strong callosity for the attachment of the cardinal muscle. 
I refer this species to the 0. lenticularis of Yanuxem with some little doubt, 
since the original specimen in the State Collection has been lost. The form de¬ 
scribed by Mr. Yanuxem is similar to that of figs. 1 and 2 of Plate y, which is 
subquadrate, the front being sometimes rounded and sometimes straight. The 
other forms ( figs. 3, 4 and 5), which I find myself compelled to refer to the same 
species, are more robust, with the greatest width below the middle, and a much 
greater convexity of both dorsal and ventral valves. 
In form, this species resembles the 0. subcarinata of the Lower Helderberg 
group; but the striae are finer, the area larger, and the muscular callosity and 
dental lamellae are stronger; while the narrowing towards the hinge in the more 
robust forms is a character not observed in that species. 
The specimens ordinarily seen have a length and breadth of a little more or 
less than half an inch, with a depth of about one quarter of an inch ; while the 
largest specimens have a length and breadth of three-fourths of an inch, with a 
depth of about half an inch. 
Geological formation and locality. In the Corniferous limestone of the Upper 
Helderberg group : at Leroy, Genesee county; near Caledonia, Livingston 
county; tAvo miles south of Yienna, and at Shortsville, Ontario county, N.Y. 
B. Species of the type of Orthis hybrida and O. oblata, with the dorsal valve more 
convex than the ventral : 
Orthis alsus, Orthis ( sp. ? ), 
Orthis mitis, Orthis vanuxemi? 
Orthis livia, Orthis semele, 
Orthis cleobis. 
Oithis alsus ( n. s.). 
PLATE IY. 
Shell somewhat semielliptical in outline : cardinal line equal to, or 
more than two-thirds the greatest width, which is near or just below 
the middle. Dorsal valve more convex than the ventral, the greatest 
convexity being about the middle, sloping almost equally to the sides 
and front, and less rapidly to the beak, while it is flattened or concave 
towards the cardinal angles. The centre is marked by a sinus from the 
